Frequency discriminator for carrier shift signaling systems and the like



Aug. 2, 1949. R. s. CHAPIN 2,477,963 FREQUENCY DISCRIMINATOR FOR CARRIER SHIFT SIGNALING SYSTEMS AND THE LIKE Filed Feb. e, 1946 2 Sheets-Sheet 1 Aug. 2, 1949. R. s. CHAPIN 2,477,963 FREQUENCY DISCRIMINATOR FOR CARRIER SHIFT v SIGNALING SYSTEMS AND THE LIKE Filed Feb. 6, 41946 2 sheets-sheet 2 oar/ 07 0/- 0/00: a;

Kf/A/DLD IN VEN TOR.

TOR/V57 Patented Aug. 2, 1949 FREQUENCY DISCRIMI NATOR FOR CARRIER SHIFT SIGNALING SYSTEMS AND THE LIKE Reynold S. Chapin, Flushing, N. Y., assignor to Press Wireless, Inc., New York, N. Y., a corporation of Delaware Application February 6, 1946, Serial No. 645,917 4 Claims. (Cl. 250-27) This invention relates to wave signalling systems and more particularly to improvements in carrier frequency shift signalling systems.

A principal object of this invention is to provide an improved tuning indicator for controlling the in-tune condition of frequency shift telegraph apparatus and the like.

A feature of this invention criminator and detector combination having components so correlated and arranged as to provide a resultant characteristic having a pair of symmetrical linear portions which equally diverge on opposite sides of a center or zero frequency.

Another feature relates to an electronic discriminator and associated grid-controlled electron tube circuits wherein any out-of-tune or undesirable frequency-shift condition results in potential changes of like magnitude and unlike polarity at both ends of the shift range.

Another feature relates to a tuning indicator arrangement for frequency shift systems employing a pair of balanced discriminators connected to a common indicator device, one discriminator having a sensitivity response which is a multiple of the other discriminator.

A further feature relates to a discriminator detector null type indicator combination wherein the indication of the tuning condition of the receiver is independent of the keying condition.

A. still further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved tuning control circuit for carrier frequency shift receiving systems.

Other features and advantages not particularly enumerated will be apparent after a consideration of the following detailed descriptions and the appended. claims.

In the drawing, r

Fig. 1 shows in schematic form a null type frequency shift deviation and tuning indicator arrangement embodying the inventive features.

Fig. 2 shows the voltage characteristics ofjthe embodiment shown in Fig. l at each of the two discriminators and the resultant voltage characteristic at the input to the indicator.

tively.

Fig. 3 shows in schematic form a modification of the embodiment shown in Fig. 1.

Fig. 4 is a schematic block diagram of the system of Fig. 1.

When signalling is effected by a frequency shifting of a radio carrier, the radiated wave in the case of telegraph transmission consists of a markrelates to a dis- Q These char,-. acteristics are designated as a, b, and respec ing frequency and a spacing frequency. When. such systems are provided with automatic frequency control or tuning indicators to keep the.

receiver in the in-tune condition, separate control channels are usually required for each of the two conditions corresponding respectively to mark and space. This has usually been necessary because of the frequency versus voltage character-f istic of the usual discriminators which have only a single crossover with the output axis regardless of whether the system is fin tune. or out-oftune. This single crossover corresponds to the.

mean frequency of the limits between which the carrier is shifted for mark and space. of the carrier frequency causes a change in detected output. With respect to the reference potential axis the detected output increases during one of the keying signals and decreases during In my prior application Serial No. 615,838,

filed September 12, 1945, I disclosed a frequency discriminator and detector arrangement having the necessary equal change of output at the mark and space frequencies. This prior arrangement requires a special form of electronic or electromagnetic switch for controlling the required tuning indications to indicate the amount of off I tune of the receiver.

Referring more particularly to Fig. 4, the antenna l0 conveys frequency shifted carrier waves to any suitable type radio receiver H. The output of receiver I l is fed to the parallel connected discriminators l3- and [4 which may be 'of any suitable type such as a linear characterist dual diode discriminator. The output characteristic" of discriminator I3 is shown by curve a (Fig. 2) and the output characteristic of discriminator I4 is shown by curve I) (Fig. 2). Relative to the zero potential axis the positive portions of these curves correspond to the mark signal voltage, and the negative portions correspond to the space signal voltage. Discriminator I4 is distinguished from discriminator l3 in that it has twice the; voltage sensitivity. This may be appreciated by A drifting 3 comparing the relative voltage values represented by curves (1 and b.

The output of discriminator I4 is conveyed to a suitable detector 35 which may comprise a diode electron tube. Detector 35 suppresses or eliminates the positive portion of the output of discriminator l4. This portion is shown by the dotted portion of curve b while the solid portion represents the detector output.

The outputs of discriminator l3 and detector 35 are fed into a voltage sensitive mixing or combining network 32 which produces an output equal to the arithmetic sum of the two discriminator outputs. The electron tube 32 may be of the linear triode type having the output of discriminator l3 impressed upon its control grid and the output of detector 35 impressed upon its cathode, thereby causing the latter output to suffer a phase reversal and the arithmetic sum of the two outputs impressed thereon to be produced at the anode of the tube. The output of network 32 for the in-tune condition of receiver H therefore provides mark and space voltages of like polarity and equal magnitude. This balanced output of network 32 is represented by curve (Fig. 2) and is fed into the deviation and tuning indicator 34 which may be of any suitable type such as a current sensitive meter. It follows from curve 0 that for the in-tune condition the current flowing through indicator 34 is proportional to the frequency difference between the mark and the space signal. Indicator 34 may be calibrated to give a direct indication of the frequency deviation of the actual mean vaiue of the mark and space frequencies.

Referring to Fig. 1, antenna ID receives frequency shifted carrier waves from any wellknown type of carrier frequency shift transmitter. If the carrier wave has a mean frequency F5, it may be shifted in one direction to a frequency of Fm to represent a mark signal and in the opposite direction to a frequency of F5 to represent a space signal For a detailed description of a typical carrier frequency shifting arrangement, reference may be had to application Serial .No. 498,278, filed August 12, 1943 now U. S. Patent No. 2,337,098. The received carrier waves are conveyed from antenna to any suitable type of radio receiver ll- The audio frequency output of receiver .H is conveyed to any conventional type of peak limiter [2 which limits the signal amplitude to a maximum fixed value.

The peak limited signals from limiter [2 are fed to the parallel connected balanced linear fre quency discriminator circuits J3 .and 14. These circuits comprise similar dual diodes l5 .and I6, inductanoes .l] and I8, .19 and variable condensers 2| and 22, resistors 23 and .24, 25 and 26,

and condensers 2.! and 28., 2-9 and .30, respectively connected in balanced relationship.

The discriminator voltage characteristics a and .b shown in Fig. 2 represent the output voltage characteristic .of discriminator circuits 1.3 and 44 respectively. F5 refers to the space sig-- nal amplitude at the discriminator output; F0 the mean frequency amplitude; and Fm the mark signal amplitude. The output characteristic a of discriminator l3 relative to F5 consists of a positive and negative output voltage which corresponds to the mark and space signals respectively. This output is fed directly to control grid 34 'of lineartniode 32. Plate 33 of tube 32 is connected to the current sensitive tuning indicator 34. Indicator '34 need not be a grid-controlled tube, but may be any suitable voltage sensitive device, such for example as a lamp, a so-called fluorescent target tuning indicator tube and the like.

Discriminator I4 is similar in design to discriminator 13 but has twice the sensitivity. The discriminator voltage characteristic b of discriminator I4 is of the same polarity as the characteristic a of discriminator l3, and has the same frequency range and double the voltage sensitivity. The output .of discriminator I4 is conveyed to the diode tube 35. The output of tube 35 is represented by the Solid portion of curve b (Fig. 2) and appears across the adjustable load resistor 36. Resistor 36 is connected to cathode 31 of tube 32, through adjustable resistor 38. Cathode .31 is connected to ground through the adjustable cathode resistor 39. Proper adjustment of the adjustable resistors 36, 38 and 39, causes the mark and space signals to produce equai currents through the voltage sensitive tube 32. The resultant voltage characteristic of that tube is linear and. the space voltage and mark voltage are of like polarity. By combining the output a of discriminator l3 and the output b of diode tube 35 by connection respectively to the control grid and cathode of tube 32, the resultant of the space voltage F5 (curve a) of discriminator I3, and the space voltage F5 (curve 1)) of diode 35, is made equal in magnitude and of like sign to the mark voltage Fm of discriminator 43. Such a condition will exist only for the in-tune condition. When so operated, the tuning indicator 34, does not fluctuate with the keying signal. This characteristic is of special advantage when it is desired to tune to a transmitted signal of unknown frequency deviation. From the resultant voltage characteristic (2, it follows that the amplitude of the current through tube 32 is proportional to "the frequency difference between the mark and space signals. When the receiver is in-tune, the indicated deflection of indicator 34 is .a measure of the deviation of the mark signal Fm and space signal F5 from the mean frequency F5. By proper calibration of meter 34 a direct indication of the deviation of the actual mean frequency at any given time is obtained.

It is sometimes necessary to tune to a transmitted signal which has an unknown frequency difference between the mark signals and space signals. The indication produced by device 34 of this invention is unaffected by changes in the actual frequency difference between those signals. This enables the operator to tune to a transmitted signal of unknown frequency difference between the mark and space signals by tuning the receiver until the tuning indicator ceases to fluctuate with the signal keying.

When detuning of the receiver occurs, the sum of the two voltages above referred to as corresponding to F5 are not equal to the Fm Voltage at the output of discriminator l3. This causes a deflection .of meter 34 which increases as the detuning increases.

The embodiment shown in Fig. 3, uses a single balanced discriminator circuit. The principle of operation of that circuit is the same as the dual discriminator circuits described in connection with Fig. 1. The balanced discriminator circuit includes .diode tubes 40 and 4| which receive signals through transformer 42 from a source such as described in connection with Fig. 1. The discriminator circuit also includes condensers 21, 29, and 42a, resistors '23 and 25 and the feedback coupling condenser 43. Resistor 44 and adjustable resistor 45 are connected to the control grid 3| of the linear triode amplifier tube 32.

The platecircuit of diode 35 contains a resistor 36 having a resistor 38a connected between the adjustable contact arm 36a and the cathode 31.

The values of resistors 44, 45 and 3B are such that when potentiometer arms 36a and 45b are properly adjusted, the amplitude of the negative signals across resistor 36 is more than twice the amplitude of the negative signals applied to control grid 3|. By proper choice of resistor 38a and proper adjustment of adjustable resistors 36 and 39, the currents flowing from plate 33 of tube 32 into indicator 34 are made equal for the positive and negative signal pulses appearing at the output of the discriminator circuit. Indicator meter 34 is calibrated to read frequency deviation for the-flow of these equal currents which indicates that the receiver is in the in-tune condition.

While there has been here described specific embodiments, various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A frequency discriminating system for an input frequency spectrum comprising a pair of separate frequency discriminator channels, means to impress said spectrum in parallel on said networks, means to adjust the frequency response sensitivity of one channel so that it is a multiple of the other, and means to combine the output of said discriminator channels to produce a result ant characteristic having two oppositely sloped linear sections of the same polarity which symmetrically diverge from a predetermined mean frequency.

2. A frequency discriminating system according to claim 1 in which the means for combining the outputs of said discriminator channels includes a grid-controlled electron tube, one discriminator channel being connected to bias the control grid of said tube and the other discriminator channel being connected to bias the cathode of said tube.

3. A frequency discriminator arrangement comprising a plurality of discriminator channels one of which has circuit elements to impart thereto a sloped linear characteristic portion relating output voltage to input frequency between two predetermined frequencies F1, F2, the other channel having circuit elements to impart thereto a sloped linear characteristic portion relating output voltage to input frequency between said two frequencies, said circuit elements being proportioned so that the slope of one discriminator characteristic is greater than the slope of the other discriminator characteristic, and means to combine the outputs of said channels to provide an overall characteristic having two oppositely and equally sloped linear portions symmetrically diverging from the mean frequency between F1 and F2 at which the output is at a minimum.

4. A frequency discriminator arrangement according to claim 3 in which the said circuit elements are proportioned so that the slope of one discriminator characteristic is substantially twice that of the other discriminator characteristic.

REYNOLD S. CHAPIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,243,414 Carlson May 27, 1941 2,272,052 Reid Feb. 3, 1942 2,286,410 Harris June 16, 1942 2,333,990 Dome Nov. 9 1943 2,353,468 Holst et al. July 11, 1944 2,379,764 Thomas July 3 1945 2,397,884 Rhodes Apr. 2, 1946 

